As a cost effective and therefore interesting alternative to conventional solar modules of crystalline silicon, thin-film solar cells have become known. The substantial elements of a thin-film solar cell consist of a thin-film stack on a glass substrate, the thin-film stack essentially comprising an absorber layer sandwiched between a back electrode and a front electrode. The thin-film-stack needs to be encapsulated so that the delicate thin film materials do not come in contact with the outside environment for both electrical safety reasons and also to prevent deterioration with time when subjected to weather. It is usual to do this by bonding a second piece of glass to the coated side of the substrate. In order that there is no electrical path to the outside edge of the panel and to ensure that the bond between the 2 layers of glass is strong and durable it is necessary to remove the stack of thin films in a border region around the panel edge.
There are different methods known in the art to remove the thin-film stack from the border region, such as sand blasting methods and laser ablation methods.
The sand blasting methods are unreliable and costly as many of the thin-films use materials that require special disposal techniques (eg. CdTe). The recovery of these materials from the sand is difficult and therefore expensive.
Laser ablation techniques are advantageous as no mixture of ablation medium and ablated material is created and recovery of the thin-film materials therefore is no issue.
In U.S. Pat. No. 4,734,500 a laser ablating method is disclosed based on a laser ablation apparatus. Basic elements of such a laser ablation apparatus are a laser oscillator, an optical unit comprising optical elements for manipulating laser light and a table for placing and holding the substrate. The laser oscillator typically emits short pulses of laser light to be used for ablation. The optical elements are used for propagating the light pulses to the surface of the substrate, which is placed on the table. During the ablation process the produced laser light needs to be moved over the surface of the substrate to be machined. Therefore a relative movement of laser beam and the substrate is required. This may be achieved by either moving the substrate with respect to stationary optics or by moving the optics unit with respect to the stationary panel or a combination, using for example CNC X and Y stages.
FIG. 1 in U.S. Pat. No. 4,734,550 shows an embodiment where an XY-table is used to move the substrate and the optics unit is stationary. In this case the substrate is maintained in a horizontal orientation with the film side upwards during ablation. This leads to the possibility that for example due to gravity ablated material will be incompletely extracted and will be re-deposited on the surface of the workpiece.
FIG. 12 in U.S. Pat. No. 4,734,550 shows an embodiment where the “table” is vertically suspended from a carry rail. The optical unit is collectively mounted on an X-Y plotter and therefore moving with respect to at least in one direction stationary substrate. At least for ablation, which is performed on the upper part of the workpiece, for example when the thin-film material in a border region along the upper edge is to be removed—this leads to the possibility that ablated material will be incompletely extracted and will be re-deposited on the surface of the workpiece.
Therefore there is a need for a laser ablation apparatus and method which overcomes or at least minimizes the problem of re-deposition.